Transmission level setting circuit and modem unit using the same

ABSTRACT

A transmission level setting circuit outputs a signal having a transmission level with respect to a portable telephone based on a signal transmitted from a modem unit. The transmission level setting circuit includes a part which receives the signal from the modem unit, and a circuit which automatically outputs a signal having a fixed level regardless of the signal transmission level of the modem unit.

BACKGROUND OF THE INVENTION

The present invention generally relates to transmission level settingcircuits and modem units, and more particularly to a transmission levelsetting circuit which sets a carrier transmission level when making adata communication on a switched line or a radio line of a portabletelephone, and to a modem unit using such a transmission level settingcircuit.

Recently, it has become possible to make a data communication using aswitched line or a radio line of a portable telephone by connecting amodem unit to the portable telephone or the switched line andcontrolling the modem unit from a personal computer or the like. In thiscase, because of the structural differences between the switched lineand the radio line, the methods of setting the carrier transmissionlevel from the modem unit are different between the case where theswitched line is used and the case where the radio line is used. Forthis reason, in the general modem unit, the setting of the carriertransmission level is changed depending on the line used.

When making the data communication via the switched line, signalattenuation occurs depending on the distance from the modem unit to anexchange. Hence, in order to set the carrier transmission level of themodem unit to an optimum value, it is necessary to take intoconsideration the attenuation value and to increase and set the carriertransmission level depending on the attenuation value.

In a conventional modem unit that is used to make the data communicationvia the switched line, a standard setting of the carrier transmissionlevel when the modem unit is shipped from a factory is -15 dBm, forexample. For this reason, when this modem unit is connected to thepersonal computer or the like, it is necessary to increase the carriertransmission level at a transmitter part within the modem unit dependingon the signal attenuation value that is dependent on the distance fromthe modem unit to the exchange. Such a change in the carriertransmission level is made manually from the personal computer or thelike to which the modem unit is connected, by a qualified person such asthe engineer in charge of the work.

On the other hand, when making the data communication via the radio lineof the portable telephone, there is no need to take into considerationthe signal attenuation value because the modem unit and the portabletelephone are connected within a short distance. Hence, the carriertransmission level of the modem unit can be set to the optimum value bysetting the carrier transmission level to a maximum value within a rangeof the input level specified by the portable telephone.

A conventional modem unit that is used to make the data communicationvia the radio line of the portable telephone is connected to theportable telephone via an attenuator which reduces the standard carriertransmission level that is set at the time of the shipping from thefactory to -51 dBm, for example, and a resistor having a transmissionimpedance of 600 Ω. Accordingly, when making the data communication viathe radio line of the portable telephone, it is necessary to change thecarrier transmission level manually from the personal computer or thelike to which the modem unit is connected, by a qualified person such asthe engineer in charge of the work.

According to the conventional modem units, it was necessary to manuallychange the carrier transmission level depending on the kind of line thatis used for the data communication, and there was a problem in that itrequired a troublesome operation of manually changing the carriertransmission level. In addition, a person who may perform such atroublesome operation was limited to a qualified person such as theengineer in change of the work, thereby leading to another problem inthat the general users were prevented from freely making the datacommunication using the portable telephone.

On the other hand, it is conceivable to connect between the modem unitthat is connected to the switched line and the portable telephone anadapter having a circuit which attenuates a signal in a signal path fromthe modem unit to the portable telephone by a fixed attenuation value of36 dB, for example. Even in this case, however, problems similar tothose of the conventional modem unit described above occur because thecarrier transmission level from the adapter to the portable telephonewill change if the carrier transmission level from the modem unitchanges.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providea novel an useful transmission level setting circuit and modem unit, inwhich the problems described above are eliminated.

Another and more specific object of the present invention is to providea transmission level setting circuit which can set a carriertransmission level to an optimum level depending on the kind of linethat is used for a data communication without having the user to beaware of the kind of line used and without the need to manually changethe carrier transmission level, and to a modem unit which uses such atransmission level setting circuit.

Still another object of the present invention is to provide atransmission level setting circuit which outputs a signal having atransmission level with respect to a portable telephone based on asignal transmitted from a modem unit, comprising means for receiving thesignal from the modem unit, and a circuit, coupled to the means,automatically outputting a signal having a fixed level regardless of thesignal transmission level of the modem unit. According to thetransmission level setting circuit of the present invention, it ispossible to automatically set the signal transmission level to anoptimum level depending on the kind of line that is used for the datacommunication, without having the user to be aware of the kind of lineand without the need to manually change the signal transmission level.In addition, when making the data communication using the radio line ofthe portable telephone, the signal having the optimum fixed level forthe radio line is constantly output regardless of the signaltransmission level of the modem unit.

A further object of the present invention is to provide a transmissionlevel setting circuit comprising a first circuit which outputs a signalhaving a transmission level with respect to a switched line based on asignal transmitted from a modem unit, a second circuit which outputs asignal having a transmission level with respect to a portable telephonebased on the signal transmitted from the modem unit, and means forcontrolling the signal level transmitted from the modem unit dependingon whether a data communication is to be made via the switched line or aradio line of the portable telephone, where the means automaticallycontrols the modem unit so that the second circuit outputs a signalhaving a fixed transmission level when making the data communication viathe radio line of the portable telephone. According to the transmissionlevel setting circuit of the present invention, it is possible toautomatically set the signal transmission level to an optimum leveldepending on the kind of line that is used for the data communication,without having the user to be aware of the kind of line and without theneed to manually change the signal transmission level. In addition, whenmaking the data communication using the radio line of the portabletelephone, the signal having the optimum fixed level for the radio lineis constantly output regardless of the signal transmission level of themodem unit.

Another object of the present invention is to provide a modem unitcomprising a transmitter part which transmits a signal, and a circuitwhich automatically outputs a signal having a fixed level with respectto a portable telephone regardless of a signal transmission level of thetransmitter part, so that the modem unit outputs a signal having atransmission level with respect to a switched line and the portabletelephone. According to the modem unit of the present invention, it ispossible to automatically set the signal transmission level to anoptimum level depending on the kind of line that is used for the datacommunication, without having the user to be aware of the kind of lineand without the need to manually change the signal transmission level.In addition, when making the data communication using the radio line ofthe portable telephone, the signal having the optimum fixed level forthe radio line is constantly output regardless of the signaltransmission level of the modem unit.

Still another object of the present invention is to provide a modem unitcomprising a modem circuit part including a transmitter part whichtransmits a signal, a first circuit which outputs a signal having atransmission level with respect to a switched line based on the signaltransmitted from the transmitter part, a second circuit which outputs asignal having a transmission level with respect to a portable telephonebased on the signal transmitted from the transmitter part, and means forcontrolling the signal level transmitted from the transmitter partdepending on whether a data communication is to be made via the switchedline or a radio line of the portable telephone, where the meansautomatically controls the modem circuit part so that the second circuitoutputs a signal having a fixed transmission level when making the datacommunication via the radio line of the portable telephone. According tothe modem unit of the present invention, it is possible to automaticallyset the signal transmission level to an optimum level depending on thekind of line that is used for the data communication, without having theuser to be aware of the kind of line and without the need to manuallychange the signal transmission level. In addition, when making the datacommunication using the radio line of the portable telephone, the signalhaving the optimum fixed level for the radio line is constantly outputregardless of the signal transmission level of the modem unit.

Other objects and further features of the present invention will beapparent from the following detailed description when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing a first embodiment of a transmissionlevel setting circuit according to the present invention;

FIG. 2 is a circuit diagram showing a second embodiment of thetransmission level setting circuit according to the present invention;

FIG. 3 is a circuit diagram showing a third embodiment of thetransmission level setting circuit according to the present invention;

FIG. 4 is a flow chart for explaining the operation of the thirdembodiment of the transmission level setting circuit; and

FIG. 5 is a circuit diagram showing a fourth embodiment of thetransmission level setting circuit according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, a description will be given of a first embodiment of atransmission level setting circuit according to the present invention,by referring to FIG. 1. This first embodiment of the transmission levelsetting circuit is applied to a first embodiment of a modem unitaccording to the present invention.

In this first embodiment, a carrier transmission level from the modemunit is fixed to -51 dBm when making a data communication via a radioline of a portable telephone. On the other hand, the carriertransmission level from the modem unit is set to -7 dBm to -15 dBm whenmaking the data communication via a switched line.

In FIG. 1, the modem unit generally includes a modem circuit part 1, anautomatic gain control (AGC) circuit 2, operational amplifiers 3 and 8,resistors 4 through 7, and a line transformer 9 which are connected asshown.

The modem circuit part 1 generally includes a modem 11, a transmitterpart 12, and a receiver part 13. In FIG. 1, G indicated within the modempart 1 denotes ground. The modem circuit part 1 itself may be formed bya modem large scale integrated circuit (LSI) which is generallyavailable on the market.

In FIG. 1 and FIGS. 2, 3 and 5 which will be described later, theconnections to the receiver part 13 within the modem circuit part 1 arenot directly related to the subject matter of the present invention, andillustration and description thereof will be omitted for this reason.

The AGC circuit 2 includes operational amplifiers 21 through 23,resistors R1 through R7, diodes D1 and D2, and a capacitor C which areconnected as shown in FIG. 1. For example, the resistors R3 through R5and R7 have resistances of 20 kΩ, and the resistor R6 has a resistanceof 10 kΩ. In addition, the capacitor C has a capacitance of 10 μF, forexample.

For example, the resistors 4 and 7 have resistances of 600 Ω, and theresistors 5 and 6 have resistances of 10 kΩ. A node N1 is connected to aportable telephone 10 via a connection cable 18, and a node N2 isconnected to a switched line (not shown) via a connection cable 19.

The modem circuit part 1 is constructed to transmit a carrier having alevel of 0 to -15 dBm depending on the setting of the transmission levelfor the switched line. The operational amplifier 21 and the resistors R1and R2 form a voltage controlled amplifier circuit, and this voltagecontrolled amplifier circuit normally functions as a 0 dB amplifier. Forthis reason, the voltage controlled amplifier circuit transmits acarrier having a level of 0 to -15 dBm depending on the level setting ofthe modem circuit part 1. The carrier output from the voltage controlledamplifier circuit is subjected to a full-wave rectification by theoperational amplifiers 22 and 23, and is smoothened by the capacitor Cwhich is connected to the operational amplifier 23. Hence, an output ofthe operational amplifier 23 corresponds to an A.C. voltage averagevalue of the output waveform of the voltage controlled amplifiercircuit.

The A.C. voltage average value output from the operational amplifier 23is supplied to a control signal input terminal of the voltage controlledamplifier circuit to form a feedback loop. Accordingly, the attenuationvalue of the voltage controlled amplifier circuit becomes large when theA.C. voltage average value is large. On the other hand, the A.C. voltageaverage value becomes small when the output of the voltage controlledamplifier circuit becomes small, and the feedback loop assumes abalanced state when the output level of the voltage controlled amplifiercircuit becomes a constant value.

The output level of the voltage controlled amplifier circuit in thebalanced state of the feedback loop can be set constantly to -9 dBm byappropriately selecting the resistances of the resistors R1 and R2.Thus, the output of the voltage controlled amplifier circuit having thelevel of -9 dBm is attenuated to 36 dB by the operational amplifier 3,and the carrier transmission level via the resistor 4 and the node N1 isconstantly set to -51 dBm. In other words, regardless of the setting ofthe transmission level of the modem circuit part 1 for the switchedline, the carrier transmission level from the node N1 is constantly andautomatically set to a fixed value of -51 dBm in this embodiment.

On the other hand, the carrier transmitted from the transmitter part 12of the modem circuit part 1 and having the level of 0 to -15 dBm isoutput from the node N2 with respect to the switched line via anamplifier circuit which is made up of the operational amplifier 8 andthe resistors 5 and 6, the resistor 7 and the line transformer 9. Forthis reason, the carrier transmission level from the node N2 is set to-7 dBm to -15 dBm depending on the transmission level of the modemcircuit part 1 for the switched line. The transmission level of themodem circuit part 1 for the switched line is set manually from a hostunit (not shown) such as a personal computer via a modem unit controller(not shown) which is connected to the modem circuit part 1 by aqualified person such as the engineer in charge of the work, similarlyas in the conventional case.

As a result, according to this embodiment, there is no need to manuallychange the carrier transmission level depending on the kind of line thatis used for the data transmission, and it is unnecessary to carry outthe troublesome operation of manually changing the carrier transmissionlevel.

Next, a description will be given of a second embodiment of thetransmission level setting circuit according to the present invention,by referring to FIG. 2. This second embodiment of the transmission levelsetting circuit is applied to a second embodiment of the modem unitaccording to the present invention. In FIG. 2, those parts which are thesame as those corresponding parts in FIG. 1 are designated by the samereference numerals, and a description thereof will be omitted.

In this embodiment, the carrier transmission level from the modem unitis fixed to -51 dBm when making the data communication via the radioline of the portable telephone. On the other hand, when making the datacommunication via the switched line, the carrier transmission level fromthe modem unit is set to -7 dBm to -15 dBm.

A modem unit shown in FIG. 2 generally includes the modem circuit part1, the operational amplifiers 3 and 8, the resistors 4 and 7, the linetransformer 9, photocouplers 121 and 122, and an AND circuit 23 whichare connected as shown. The photocouplers 121 and 122 are provided in asignal path to the switched line, so as to detect whether or not a D.C.current flows through this signal path. When no D.C. current flowsthrough this signal path, the modem 11 of the modem circuit part 1automatically sets the carrier transmission level of the transmitterpart 12 to a fixed value of -9 dBm based on a set signal from the ANDcircuit 23. Hence, the output of the transmitter part 12 having thelevel of -9 dBm is attenuated to 36 dB in the operational amplifier 3,and the carrier transmission level via the resistor 4 and the node N1 isconstantly set to -51 dBm. In other words, regardless of the setting ofthe transmission level of the modem circuit part 1 for the switchedline, the carrier transmission level from the node N1 is constantly andautomatically set to the fixed value of -51 dBm in this embodiment whenno data communication is made via the switched line. That is, whenmaking the data communication via the radio line of the portabletelephone, the carrier transmission level from the node N1 is constantlyand automatically set to the fixed value of -51 dBm.

On the other hand, the carrier which is transmitted from the transmitterpart 12 of the modem circuit part 1 and has the level of 0 to -15 dBm isoutput from the node N2 with respect to the switched line via theoperational amplifier 8, the resistor 7 and the line transformer 9. Forthis reason, the carrier transmission level from the node N2 is set to-7 dBm to -15 dBm depending on the setting of the transmission level ofthe modem circuit part 1 for the switched line. The transmission levelof the modem circuit part 1 for the switched line is set manually from ahost unit (not shown) such as a personal computer via a modem unitcontroller (not shown) which is connected to the modem circuit part 1 bya qualified person such as the engineer in charge of the work, similarlyas in the conventional case. In addition, when making the datacommunication via the switched line, a D.C. current flows through thesignal path to the switched line, and no set signal is supplied from theAND circuit 23 to the modem 11 of the modem circuit part 1. Accordingly,the operation of automatically setting the carrier transmission levelfrom the transmitter part 12 to the fixed level of -9 dBm is not carriedout in this case.

As a result, according to this embodiment, there is no need to manuallychange the carrier transmission level depending on the kind of line thatis used for the data transmission, and it is unnecessary to carry outthe troublesome operation of manually changing the carrier transmissionlevel.

Next, a description will be given of a third embodiment of thetransmission level setting circuit according to the present invention,by referring to FIG. 3. This third embodiment of the transmission levelsetting circuit is applied to a third embodiment of the modem unitaccording to the present invention. In FIG. 3, those parts which are thesame as those corresponding parts in FIG. 1 are designated by the samereference numerals, and a description thereof will be omitted. Further,the circuit provided with respect to the switched line is identical tothat shown in FIG. 1, and thus, the illustration thereof is omitted inFIG. 3.

In this embodiment, the carrier transmission level from the modem unitis fixed to -51 dBm when making the data communication via the radioline of the portable telephone.

A modem unit shown in FIG. 3 generally includes the modem circuit part1, the operational amplifier 3, resistors 4 and 30, a microprocessorunit (MPU) 31, and a connector 41 which are connected as shown. Whenmaking the data communication via the radio line of the portabletelephone, the connector 41 is connected to a connector 42. When theconnectors 41 and 42 are connected, the transmitted carrier from thetransmitter part 12 of the modem circuit part 1 is passed through theoperational amplifier 3 and the resistor 4 and is output from the nodeN1 of the connector 42 with respect to the portable telephone 10 via theconnection cable 18. On the other hand, the resistor 30 is connectedbetween a power supply which supplies +5 V and the ground GND, and aselection signal from a node N3 is supplied to a general port P of theMPU 31.

In this embodiment, the connector 41 is provided on the side of themodem unit, and the connector 42 is provided on the side of theconnection cable 18 which connects to the portable telephone 10.However, for example, it is of course possible to provide the connector41 on the tip end of a connection cable which connects to the modemunit, and to omit the connection cable 18 and provide the connector 42directly on the side of the portable telephone 10.

The MPU 31 includes a controller 32 and a register 33. The controller 32is connected to a personal computer 45. The controller 32 receives a setvalue of the carrier transmission level of the transmitter part 12within the modem circuit part 1 and the like, and stores the set valuein an internal register of the controller 32. In addition, apredetermined fixed value for the carrier transmission level of thetransmitter part 12 is set in advance into the controller 32 from thepersonal computer 45, for example, and this predetermined fixed value isstored in the internal register of the controller 32. The controller 32stores the set value into the register 33 when making the datacommunication via the switched line and sets the predetermined fixedvalue into the register 33 when making the data communication via theradio line of the portable telephone, depending on the selection signalwhich is supplied to the general port P of the MPU 31 via the connector41. The value stored in the register 33 is read under control of thecontroller 32 and is supplied to the transmitter part 12 within themodem circuit part 1, so that the carrier transmission level of thetransmitter part 12 is set based on the value supplied from the register33.

When making the data communication via the switched line, the connector41 will not be connected to the connector 42. For this reason, the nodeN3 will not be grounded, and the controller 32 stores the set value fromthe personal computer 45 into the register 33 depending on a high-levelselection signal from the node N3. Hence, in this case, the transmitterpart 12 outputs the transmitting carrier having the level based on theset value, and the carrier having the level of -7 dBm to -15 dBm isoutput with respect to the switched line from the node N2 via a circuitsimilar to that shown in FIG. 1.

On the other hand, when making the data communication via the radio lineof the portable telephone, the connector 41 is connected to theconnector 42. For this reason, the node N3 is grounded, and thecontroller 32 stores the predetermined fixed value into the register 33based on the terminal voltage of the resistor 30 from the node N3, thatis, a low-level selection signal. Accordingly, in this case, thetransmitter part 12 automatically and fixedly outputs the transmittingcarrier having the level of -9 dBm which is based on the predeterminedfixed value, and the carrier having the fixed value of -51 dBm is outputwith respect to the portable telephone 10 from the node N1 of theconnector 42.

As a result, according to this embodiment, there is no need to manuallychange the carrier transmission level depending on the kind of line thatis used for the data transmission, and it is unnecessary to carry outthe troublesome operation of manually changing the carrier transmissionlevel. Furthermore, the setting of the carrier transmission level ismade automatically based on the connection state of the connector 41.

In FIG. 3, the MPU 31 may be a processor which forms a part of the modem11.

FIG. 4 is a flow chart for explaining the operation of the controller 32within the MPU 31 shown in FIG. 3.

In FIG. 4, a step Si writes the set value of the transmission level fromthe personal computer 45 into the internal register of the controller32. A step S2 writes a communication start command from the personalcomputer 45 into the internal register of the controller 32. A step S3decides whether the level of the selection signal supplied to thegeneral port P of the MPU 31 has a high level or a low level. When thestep S3 detects the high level, a step S4 stores the set value which isread from the internal register of the controller 32 into the register33. On the other hand, when the step S3 detects the low level, a step S5stores the predetermined fixed value which is read from the internalregister of the controller 32 into the register 33. For example, thepredetermined fixed value is -15.

After the step S4 or S5, a step S6 reads the value stored in theregister 33 and supplies the read value to the transmitter part 12. Astep S7 starts the data communication, and the process returns to thestep S2 when the end of the data communication is detected in a step S8,so as to wait for a next communication start command which is obtainedfrom the personal computer 45.

Next, a description will be given of a fourth embodiment of thetransmission level setting circuit according to the present invention,by referring to FIG. 5. This fourth embodiment of the transmission levelsetting circuit is applied to a fourth embodiment of the modem unitaccording to the present invention. In FIG. 5, those parts which are thesame as those corresponding parts in FIG. 1 are designated by the samereference numerals, and a description thereof will be omitted.

In this embodiment, the carrier transmission level from the modem unitis fixed to -51 dBm when making the data communication via the radioline of the portable telephone.

In FIG. 5, a modem unit 51 has a known construction which transmits thecarrier with respect to the switched line. Hence, the carrier having thelevel of -7 dBm to -15 dBm, for example, is transmitted from the modemunit 51. The carrier transmitted from the modem unit 51 is output withrespect to the portable telephone 10 via an adapter 52 and a three-corecable 66. This adapter 52 has a construction which constantly outputs acarrier having a fixed level of -51 dBm via the three-core cable 66regardless of the carrier transmission level from the modem unit 51.

The adapter 52 includes a transformer 61, resistors 62 through 64, andthe AGC circuit 2 which are connected as shown in FIG. 5. The AGCcircuit 2 has the same construction as the AGC circuit 2 shown inFIG. 1. A signal which is transmitted with respect to the portabletelephone 10 is output from a terminal TX, a terminal G is grounded, anda signal which is received from the portable telephone 10 is input to aterminal RX. The terminals TX, G and RX are connected to correspondinglines of the three-core cable 66.

According to this embodiment, the output signal level of the voltagecontrolled amplifier circuit in the balanced state of the feedback loopwithin the AGC circuit 2 can constantly be set to 31 9 dBm byappropriately selecting the resistances of the resistors R1 and R2 shownin FIG. 1. Hence, the output of the voltage controlled amplifier circuithaving the level of -9 dBm is attenuated into 36 dB in the operationalamplifier 3, and the carrier transmission level via the resistor 4 andthe node N1 is constantly set to -51 dBm. In other words, the carriertransmission level from the three-core cable 66 in this embodiment isconstantly and automatically set to the fixed value of -51 dBmregardless of the setting of the transmission level of the modem unit 51for the switched line.

As a result, it is unnecessary in this embodiment to manually change thecarrier transmission level depending on the kind of line that is usedfor the data communication, and there is no need to carry out thetroublesome operation of manually changing the carrier transmissionlevel. That is, it is possible to automatically set the carriertransmission level by connecting the modem unit 51 to the portabletelephone 10 via the adapter 52 only when making the data communicationvia the radio line of the portable telephone.

Of course, it is possible to provide the adapter 52 on the side of theportable telephone 10. In this case, the modem unit 51 is connected tothe adapter 52 via a connection cable when making the data communicationvia the radio line of the portable telephone 10.

Furthermore, it is possible to provide the adapter 52 on the side of themodem unit 51. In other words, in the fourth embodiment of the modemunit, the modem unit 51 integrally comprises the adapter 52. In thiscase, it is desirable to provide a selector (not shown) whichselectively outputs the output of the modem unit 51 and the output viathe adapter 52.

In other words, the AGC circuit 2 simply needs to be provided in asignal path which connects the modem unit 51 and the portable telephone10.

Further, the present invention is not limited to these embodiments, butvarious variations and modifications may be made without departing fromthe scope of the present invention.

What is claimed is:
 1. A transmission level setting circuit which sets atransmission level of a carrier output by a modem unit to apredetermined value, said transmission level setting circuitcomprising:a first circuit outputting, with respect to a switched line,a carrier having a transmission level dependent on a carriertransmission level output by the modem unit, which generates a carrierhaving a transmission level adjustable within a predetermined range; anda second circuit setting a transmission level of the carrier output bythe modem unit to a fixed transmission level corresponding to atransmission level required for a portable telephone.
 2. Thetransmission level setting circuit as claimed in claim 1, wherein saidsecond circuit comprises an automatic gain control circuit which outputsthe carrier having the fixed level regardless of the transmission levelof the carrier output by the modem unit.
 3. The transmission levelsetting circuit as claimed in claim 1, wherein said second circuit isprovided in a signal path which connects the modem unit and the portabletelephone.
 4. The transmission level setting circuit as claimed in claim1, which further comprises:a connection cable for connecting the modemunit to the portable telephone, said second circuit being providedwithin said adapter.
 5. A modem unit comprising:a transmittertransmitting a carrier having a transmission level selectable within apredetermined range; a first circuit outputting, with respect to aswitched line, a carrier having a transmission level dependent on atransmission level of the carrier output from said transmitter part; anda second circuit setting the transmission level of the carrier outputfrom said transmitter part to a fixed transmission level correspondingto a transmission level required for a portable telephone.
 6. The modemunit as claimed in claim 5, wherein said second circuit comprises anautomatic gain control circuit which outputs a carrier having a fixedlevel regardless of a carrier transmission level of said transmitterpart.
 7. The modem unit as claimed in claim 5, wherein said secondcircuit is provided in a signal path which connects the modem unit andthe portable telephone.
 8. The modem unit as claimed in claim 5, whichfurther comprises:a connection cable for connecting the modem unit tothe portable telephone, said output second circuit being provided withinsaid adapter.
 9. A transmission level setting circuit setting a dBmpower level of a carrier to a fixed transmission level specified forinput to a portable telephone, comprising:input terminals receiving acarrier having a transmission level, within a specified dBm power levelrange, from a modem; output terminals for connection to a portabletelephone; and an automatic gain control circuit, interconnecting theinput and output terminals, receiving and automatically adjusting theattenuation therein of the received carrier, for any dBm power levelthereof within the specified dBm power level range, for supply to theoutput terminals as an output carrier of the specified dBm power level.10. A transmission level setting circuit, as recited in claim 9, whereinthe automatic gain control circuit comprises:a voltage controlledamplifier circuit, functioning as a 0 dB amplifier, receiving thecarrier from the input terminals and producing a carrier output having afixed, dBm power level; and an element receiving and attenuating theoutput of the voltage controlled amplifier circuit by a fixed dB amountto produce the output carrier having a transmission level at thespecified dBm power level, at the output terminals.
 11. A transmissionlevel setting circuit, as recited in claim 10, wherein the voltagecontrolled amplifier circuit further comprises:a first operationalamplifier having a control input and producing the output carrier at anoutput thereof; and a feedback circuit producing an AC voltage averagevalue of the output carrier of the first operational amplifier, as afeedback control signal supplied to the control signal input terminal ofthe operational amplifier.
 12. A transmission level setting circuit asrecited in claim 9, wherein the output terminals comprise a first node,for connection to a portable telephone, to which the output carrier ofthe automatic gain control circuit is supplied and a second node, forconnection to a switched line having a selected dB power level range,which is different from the selected dB power level range of thereceived carrier transmission signal, further comprising:an alternativesignal path connecting the input terminals to the second node, the dBmpower signal level of the carrier received at the input terminals fromthe modem being manually adjusted for supply, through the alternativesignal path, to the second node at the selected, different dBm powerlevel range.
 13. A carrier generation apparatus, comprising:a modemproducing a carrier for transmission as an internal carrier within theapparatus having a selected one of a selectively set internal powerlevel, in a range of internal power levels, and a fixed internal powerlevel; a first circuit receiving the internal carrier produced by themodem and producing a first output carrier at a fixed, first outputpower level, based on the internal carrier but independent of theselectively set internal power level of the internal carrier; and asecond circuit receiving the internal carrier produced by the modem andselectively producing a second output carrier at a second, selectivelyset output power level in a second range of output power levelsdetermined in accordance with the internal power level and the range ofinternal power levels of the internal carrier.
 14. The carriergeneration apparatus as claimed in claim 13, wherein the first circuitcomprises an automatic gain control circuit.
 15. The carrier generationapparatus as claimed in claim 13, wherein the first circuit comprises:anautomatic gain control circuit receiving the internal carrier producedby the modem and outputting a second internal carrier based on thereceived internal carrier and having a second, fixed internal powerlevel within the range of internal power levels but independent of anyselectively set internal power level of the internal carrier produced bythe modem; and an attenuator receiving the second internal carrier,output by the automatic gain control circuit, and producing the firstoutput carrier at the fixed, first output power level, based on thereceived, second internal carrier and the second, fixed internal powerlevel.
 16. The carrier generation apparatus as claimed in claim 13,wherein the modem further comprises a control signal input terminalconnected to an external host unit for receiving a control signal fromthe external host unit, the control signal selectively setting theinternal power level of the internal carrier produced by the modem. 17.The carrier generation apparatus having a first output terminal forconnection to a portable telephone and a second output terminal forconnection to a switched line and comprising:a modem producing a carrierfor transmission as an internal carrier within the apparatus and, inresponse to the presence of data communications on the switched line,selectively setting an internal power level of the internal carrier in arange of internal power levels and, in response to the absence of datacommunications over the switched line, setting the internal carrier to afixed internal power level, in the range of internal power levels butindependently of any internal power level of the internal carrierselectively set by the modem for data communications on the switchedline; a first circuit receiving the internal carrier having the fixedinternal power level produced by the modem and producing, at the firstoutput terminal for supply to a portable telephone, a first outputcarrier at a fixed, first output power level corresponding to the fixedinternal power level; and a second circuit receiving the internalcarrier produced by the modem and producing, at the second outputterminal for supply to the switched line, a second output carrier at asecond, selectively set output power level in a range of output powerlevels, determined respectively in accordance with the selectively setinternal power level and the range of internal power levels of theinternal carrier.
 18. A carrier generation apparatus, as claimed inclaim 17, further comprising a current sensor, responsive to the absenceof current flow in the second circuit corresponding to the absence of asecond output carrier being produced by the second circuit, forsupplying a control signal to the modem for producing the internalcarrier at the fixed internal power level.
 19. A carrier generationapparatus as claimed in claim 17, further comprising:a connectorconnecting a portable telephone to the first output terminal; and adetector detecting the connection of the portable telephone to the firstoutput terminal for selectively switching the modem to produce theinternal carrier at the fixed internal power level.
 20. A transmissionlevel setting circuit setting a dBm power level of a carrier to aspecified transmission level, comprising:input terminals receiving acarrier having a transmission level, within a specified dBm power levelrange, from a modem; first output terminals for connection to a switchedline; second output terminals for connection to a portable telephone; anattenuation circuit interconnecting the input terminals and the firstoutput terminals and outputting, with respect to the switched line, acarrier having a transmission level dependent on the transmission levelof the carrier received by the input terminals; and an automatic gaincontrol circuit, interconnecting the input terminals and the secondoutput terminals, receiving and automatically adjusting an attenuationtherein of the received carrier, for any dBm power level thereof withinthe specified dBm power level range, for supply to the second outputterminals as an output carrier of the specified dBm power level.
 21. Atransmission level setting circuit, as recited in claim 20, wherein theautomatic gain control circuit comprises:a voltage controlled amplifiercircuit, functioning as a 0 dB amplifier, receiving the carrier from theinput terminals and producing a carrier output having a fixed, dBm powerlevel; and an element receiving and attenuating the output of thevoltage controlled amplifier circuit by a fixed dB amount to produce theoutput carrier having a transmission level at the specified dBm powerlevel, at the second output terminals.
 22. A transmission level settingcircuit, as recited in claim 20, wherein the voltage controlledamplifier circuit further comprises:a first operational amplifier havinga control input and producing the output carrier at an output thereof;and a feedback circuit producing an AC voltage average value of theoutput carrier of the first operational amplifier, as a feedback controlsignal supplied to the control signal input terminal of the operationalamplifier.